The present invention describes a process for preparing alcoholic solutions of alkali metal alkoxides in microreactors.
Alkali metal alkoxides are very important as intermediates, reactants and catalysts in the synthesis of many organic compounds. Various processes are known for making them.
The alkali metal alkoxides of tertiary alcohols have wakened growing interests for use as catalysts and condensation agents, since the associated alcohols are sterically hindered, have low acidity and hence are stronger proton acceptors and substantially less prone to side reactions than primary or secondary alcohols. In many cases the alkoxides are used in the form of their solutions in the corresponding alcohol. The alcohol can also act as a solvent in the desired reaction, so that the number of solvents used can be reduced, providing economical processes involving less equipment. In addition, in the form of their solutions the alkoxides are substantially less problematical to handle on an industrial scale than in solid form.
One of the ways to prepare alkoxides is to react the free alkali metal with the alcohol directly. It is known, for example from DE-A-26 12 642, that the reaction slows with increasing chain length of the alcohol and also increasing degree of branching: primary alcohols give the fastest reaction, tertiaries the slowest. Various specific processes for preparing alkoxides are known from the literature.
DE-A-23 33 634, DE-A-26 12 642 and EP-A-0 749 947 disclose processes for preparing alcohol-free alkali metal alkoxides in an inert solvent by batchwise reaction of the alkali metal with the alcohol. DE-A-23 33 634 utilizes elevated temperature and pressure to speed the reaction, while DE-A-26 12 642 and EP-A-0 749 947 utilize the sodium metal in finely divided form. However, all these processes have the disadvantage that an additional inert solvent is used. To obtain an alcoholic solution of the alkali metal alkoxide, this inert solvent must first be removed again completely and the solid alkoxide obtained redissolved in the alcohol. This creates additional costs, more equipment is needed and process times lengthened.
EP-A-0 192 608 discloses a batch process for preparing alcoholic solutions of alkali metal tert-alkoxides by reacting an alkali metal with a tertiary alcohol by adding the hot alcohol to the molten alkali metal while stirring with an anchor or blade agitator. Again, the reaction takes hours. Moreover, alcohols having boiling points below the melting point of the alkali metal used cannot be reacted at a sufficient rate, since the solid alkali metal cannot be adequately dispersed in the solvent.
WO 99/65849 discloses a batch process for preparing solid alkali metal alkoxides in the presence of a catalyst which, however, is not removed thereafter.
A feature common to these processes is the relatively large amount of alkali metal which has to be used all at once and which is contacted with alcohol, so that substantial amounts of hydrogen are also formed within a short time. This leads to high safety requirements on an industrial scale. To obtain economically sensible reaction times, the fine division of the alkali metal is decisive, and it is usually obtained by intensive mechanical dispersion. Similarly, taking processes from the laboratory scale to the large industrial scale is inconvenient with batch processes and can present problems, since for example vessel and stirrer geometries or heat transfers have a substantial effect on reaction times and conversion rates, for example.
It is an object of the present invention to provide a process for economical, technically reliable and rapid preparation of alcoholic solutions of alkali metal alkoxides that leads to economical reaction times and satisfactory production scale processes even in the case of higher and branched alcohols in that scale-up is simple to accomplish and the process safety risk can be minimized, for example by minimizing the amount of alkali metal added per reactor volume.
It is known to conduct certain chemical reactions in microreactors. Microreactors are constructed from stacks of grooved plates and described in DE 39 26 466 C2, U.S. Pat. No. 5,534,328 and U.S. Pat. No. 5,811,062.
It has now been found that, surprisingly, microreactors are useful for preparing alcoholic solutions of alkali metal alkoxides.
As used herein, the term xe2x80x9cmicroreactorxe2x80x9d is representative of micro- and minireactors, which differ only by reasons of the dimensions and construction of the reaction channel structures. It is possible to use, for example, microreactors as known from the cited references or from publications of the Institut fxc3xcr Mikrotechnik Mainz GmbH, Germany, or else commercially available microreactors, for example Selecto(trademark) based on Cytos(trademark) from Cellular Process Chemistry GmbH, Frankfurt/Main.
The invention accordingly provides a process for preparing alcoholic solutions of alkali metal alkoxides, which comprises reacting an alkali metal with an alcohol in a microreactor.